Minimally invasive interventions in the heart, e.g. catheter ablations and the placement of stents are nowadays generally controlled with the aid of fluoroscopic images. These are two-dimensional (2D) x-ray images recorded in real time, which are preferably obtained by means of special interventional C-arm x-ray devices. Being radioscopic images, the fluoroscopic images show no spatial-depth information, but they are available in real time and minimize the radiation loading for patient and doctor.
The idea is already emerging of supplementing the missing spatial-depth information by registering a pre-operatively recorded three-dimensional (3D) image data set of the heart with the two-dimensional fluoroscopic images and then representing the images in combination with one another, that is e.g. overlaid. The pre-operative 3D-image data set can be obtained by means of computer tomography (CT), magnetic resonance tomography (MR) or by means of 3D angiography e.g. by means of a rotational pass with a C-arm x-ray device. The combination of 2D and 3D images registered with one another then allows the doctor to obtain better orientation in the image volume.
Such a method is known from DE 102 10 646 A1. Here, a method is described for the combined representation of a series of consecutively recorded 2D fluoroscopic images of the heart, multiple 3D image data sets being recorded ECG-triggered and assigned to the corresponding 2D fluoroscopic images using the ECO. A reconstructed image of the 3D image data set is overlaid with the corresponding 2D radioscopic image and represented on a monitor.
Furthermore, DE 102 10 646 A1 also discloses a corresponding examination device, which allows registration of the 3D reconstructed images with the 2D radioscopic images by means of an image-processing apparatus.
When combining images in such a way, there are essentially two problems to be solved:
1. The Image Registration:
It must firstly be determined from which direction the 3D image volume has to be projected in order that it can be matched to the 2D fluoroscopic image. Registration is thus the determination of a transformation matrix by means of which, from the position of a voxel in the 3D image data set, the position of the voxel on the corresponding 2D fluoroscopic image can be calculated. There are various approaches to this, which will not, however, be described in detail here. Normally, various projections of the 3D image data set are calculated and compared with the 2D fluoroscopic images until a match is attained. Registration is simplified if the 3D image data set has been reconstructed from x-ray images of a rotational pass which was recorded on the same C-arm x-ray device as the fluoroscopic images. In this case, the registration can be calculated from the known equipment geometry.
2. Visualization:
The second problem is visualization of the 2D and 3D images registered with one another, i.e. the combined representation of fluoroscopic image and a corresponding projection of the 3D image data set.
The standard method of visualization is the so-called “overlay” i.e. the two images are placed over one another and made partially transparent so that they are fused with one another. This corresponds to a representation like that produced by two slide images projected onto the same screen. The proportion of the fused image that each of the two individual images makes up can be adjusted (“blending”).
This has the advantage that spatially associated image information from the 2D and 3D images is also represented visually in the same position. The disadvantage is that a static 3D image data set is overlaid with a dynamic 2D image. The acquisition of a 3D image data set is namely usually ECG-triggered at a defined cardiac phase, whereas fluoroscopic images are recorded in real time, and so are not ECG-triggered. This hampers orientation above all in the cardiac phases in which the 2D image does not coincide with the 3D image.